1. Field of the Invention
The present invention relates to 2,5-bis(1,1-dialkoxy-2-propyl)-2,5-dihydrofurans of the formula I ##STR2## where R.sup.1 and R.sup.2 can be identical or different and are alkyl of 1 to 4, preferably 1 or 2, carbons, to a simple process for the preparation thereof and to the use thereof for the preparation of 2,7-dimethyl-2,4,6-octatrienedial and of symmetric carotenoids.
2. Description of the Related Art
An advantageous process for the preparation of symmetric carotenoids which are in demand, such as the C.sub.40 -carotenoids of the formula IV .beta.-carotene, canthaxanthin and astaxanthin, is based on linkage at both ends of 2,7-dimethyl-2,4,6-octatrienedial to the appropriate C.sub.15 -triphenylphosphonium salts of the formula V in a Wittig reaction (cf. DE 28 01 908).
The C.sub.10 -dialdehyde 2,7-dimethyl-2,4,6-octatrienedial required for this purpose is prepared in the prior art from methylglyoxal dimethyl acetal and 1,4-bis(dimethylphosphono)-2-butene by a Wittig-Horner reaction and acetal hydrolysis ( cf., for example, Angew, Chem, 89 (1977) 437-443).
The processes for preparing the precursors methylglyoxal acetal and 1,4-bis(dimethylphosphono )-2-butene require elaborate safety techniques and thus make the C.sub.10 -dialdehyde a costly component in the synthesis.
Thus, methylglyoxal acetal is obtained from acetone in two stages in which first the acetone is nitrosated with methyl nitrite and then the oxime is reacted with methanol to give the required product, 1,4-Bis(dimethylphosphono )-2-butene is obtained from 2-butene-1,4-diol via 1,4-dibromo-2-butene, which is very objectionable from the health point of view, and reaction thereof with trimethyl phosphite. The byproduct is bromomethane which is likewise a health hazard.
Furthermore, it has been disclosed in CH 321 106 that the C.sub.10 -dialdehyde can also be prepared by reactions between fumaraldehyde acetals or malealdehyde acetals and alkyl propenyl ethers. The intermediate C.sub.10 -diether acetals are hydrolyzed in acid medium to the C.sub.10 -dialdehyde which can be isolated as crystals.
The disadvantage of this process is that the fumaraldehyde and malealdehyde are prepared from acetylenic precursors by partial hydrogenation. The person skilled in the art is aware that such partial hydrogenations are difficult to carry out and occur with sufficient selectivity only when exactly defined reaction conditions are strictly observed.
It is furthermore evident from CH 321 106 that the C.sub.10 -diether acetals are sensitive to heat, cannot be purified and therefore are preferably processed undistilled to the C.sub.10 -dialdehyde.
Besides the economic disadvantage of its high price, the C.sub.10 -dialdehyde has the physical disadvantage of a great tendency to crystallize, which does facilitate its isolation but subsequently interferes with its reaction with other components. Therefore large amounts of solvent and large apparatus are needed when it is used in carotenoid syntheses.